This invention relates to a touch screen that is capable of eliminating noise signals and particularly a touch screen that is capable of eliminating noise signals generated by the background light source of liquid crystal display (LCD) screen or cathode ray tube (CRT), or other external electromagnetic interference (EMI) and radio frequency interference (RFI).
Presently, voltage sensing type touch screen and current sensing type touch screen have been widely used in desk top computers, handheld computers or notebook computers. Users may write, draw pictures or select various functions or press command keys on the screen to generate electric signals and input into the computer to perform processes desired, and do not have to operate the computer through the keyboards.
Aforesaid touch screen (as shown in FIG. 1 for a traditional current sensing type touch screen) usually includes a glass layer 1, a conductive membrane layer 2, a linearization pattern layer 3, an isolation layer 4, a four wire silver printing layer 5 and a tail cable 7 connecting to a controller. The controller outputs four equal voltages to four ends of the linearization pattern of the touch screen to measure current variation.
When different point of the touch screen is touched, the current at the four ends will have different changes. Through measuring the current variation, the controller can determine the touched position. Detailed operation principle may be found in U.S. Pat. No. 4,293,734. In practical operation, the touch screen will absorb ambient noise signals into the four electric current and result in the touch screen cannot accurately respond to the touched position.
There is another type of touch screen (as shown in FIG. 2, a traditional voltage sensing type five wire touch screen) which includes a glass layer 10, an Indium Tin Oxide (ITO) conductive layer 11, a set of insulation points 12, an isolation layer 13, a four wire silver printing layer 14, another isolation layer 15, another ITO conductive layer 16, a plastic membrane layer 17 and a tail cable 18 connecting to a controller. In operating principle, the lower ITO layer links to an even electric field of 0-5V in X-axis direction. When the touch screen is touched, the upper ITO layer contacts the lower ITO layer and measures the voltage value. The voltage value ratio represents the positional ratio on the touch screen in that direction (X-axis). For instance, 3V represents the touch point located at 60% of the total length of the touch screen in the X-direction. When measuring of one direction (i.e. X-axis) is finished, the controller panel converts the upper ITO layer to an even electric field of 0-5V in Y-axis direction, then uses the lower ITO layer to measure the voltage value of touch point at the upper layer and measure the position in another direction (Y-axis). Reference details can be found in U.S. Pat. No. 3,662,105. In practical operation, the touch screen will absorb ambient noise signals into the measured voltage and result in the touch screen not being able to accurately respond to the touched position.
There is yet another type of touch screen (as shown in FIG. 3, a traditional voltage sensing type five wire touch screen) which includes a glass layer 20, an ITO conductive layer 21, a linearization pattern layer 29, a set of insulation points 22, an isolation layer 23, a four wire silver printing layer 24, another isolation layer 25, another ITO conductive layer 26, a plastic membrane layer 27 and a tail cable 28 connecting to a controller. In operating principle, the lower ITO links to an even electric field of 0-5V in X-axis direction. When the touch screen is touched, the upper ITO layer contacts the lower ITO layer and measures the voltage value. The voltage value ratio represents the positional ratio on the touch screen in that direction (X-axis). For instance, 3V represents the touch point is located at 60% of the total length of the touch screen in the X-direction. When measuring of one direction (i.e. X-axis) is finished, the controller panel converts the lower ITO to an even electric field of 0-5V in Y-axis direction, then uses the lower ITO layer to measure the voltage value of touch point at the upper layer and measure the position in another direction (Y-axis). Reference details can be found in U.S. Pat. No. 3,798,370. In practical operation, the touch screen will absorb ambient noise signals into the measured voltage and result in the touch screen not able to accurately respond to the touched position.
Although all the aforesaid traditional touch screens may enable users to operate computers without pressing button keys on the keyboards, they still have a lot of drawbacks when in use. It is because the touch screen is easily affected by the interference resulting from LCD or CRT background light source, or external EMI and RFI, and may cause not accurate sensing position and error in computer judgement or recognition. For instance, drawing a straight line on the touch screen may become a curve when displaying on the screen, or selecting A key on the keyboard map shown on the screen results in a B key displaying on the screen.
It is therefore an object of this invention to overcome the foregoing disadvantages by adding an antenna-like conductive wire on the touch screen. The conductive wire will receive same noise signals as the ones existed in the touch control signals, and the controller will use the noise signals in the conductive wire to offset the noise signals in the touch control signals for eliminating the noise signals in the control signals thereby to attain the accuracy desired.